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Review
Alternatively spliced tissue factor
A crippled protein in coagulation or a key player
in non-haemostatic processes?
Y. W. van den Berg; H. H. Versteeg
Einthoven Laboratory for Experimental Vascular Medicine, department of thrombosis and haemostasis, Leiden
University Medical Centre, Leiden, the Netherlands
Keywords
Angiogenesis, cancer, cardiovascular disease,
integrins
Summary
Full-length tissue factor (flTF) initiates coagulation, but also exerts non-hemostatic functions such as inflammation and angiogenesis
through protease activated receptors (PARs).
In 2003 a soluble variant of flTF was described
which results from alternative splicing. Since
its discovery the role of alternatively spliced
tissue factor (asTF) in coagulation has been
debated. asTF may have pro-coagulant properties but due to structural differences when
compared to flTF, asTF coagulant function
may be relatively low. Nevertheless, similar to
flTF, asTF appears to have non-hemostatic
properties; asTF expression in tumors correlates with increased tumor size, vessel
number and poor survival in some cancer
types, and drives tumor growth in animal
models. Interestingly, unlike flTF, asTF does
Correspondence to:
Henri H. Versteeg
Einthoven Laboratory for Experimental Vascular
Medicine, C2-R, Leiden University Medical Centre
P.O. box 9600, 2300 RC Leiden, the Netherlands
Tel. +31/71/526 38 72, Fax +31/71/526 67 55
E-mail: [email protected]
Full-length tissue factor (flTF) is a 47 kDa
transmembrane glycoprotein and is considered to be the principal initiator of coagulation. flTF is encoded by a mature
mRNA transcript that consists of six exons:
● Exon 1 encodes the N-terminal signal
sequence,
● exons 2–5 the extracellular domain (AA
1–219),
● exon 6 the
not promote angiogenesis through activating
PARs but rather via integrin ligation. flTF is a
critical determinant in cardiovascular disease
but little is known about asTF in cardiovascular
disease. asTF is produced by monocytes and
macrophages, thus macrophage-derived asTF
may contribute to atherosclerotic disease. In
conclusion, unraveling asTF’s non-hemostatic
properties may generate new insights in the
pathophysiology and diagnostics of cancer and
cardiovascular disease.
Schlüsselwörter
Angiogenese, Krebs, kardiovaskuläre Erkrankung, Integrine
Zusammenfassung
Der flTF (full length tissue factor) initiiert die
Gerinnung, verfügt aber durch die Protease-aktivierten Rezeptoren (PARs) auch über andere
Funktionen, wie Inflammation und Angiogenese. Im Jahr 2003 wurde eine lösliche Variante
Tissue-Faktor nach alternativem Splicing – Ein
verkrüppeltes Protein der Gerinnung oder ein
Schlüsselprotein nicht hämostatischer Prozesse?
Hämostaseologie 2010; 30: 144–149
– transmembrane
region
(AA
220–244) and
– cytoplasmic tail (AA 245–263).
Expression of flTF is normally limited to
subendothelial tissues, but upon endothelial disruption, flTF becomes exposed to
blood-borne factor VII (FVII) and then a
proteolytically active flTF:FVIIa complex is
formed. Subsequently, factor X (FX) binds
des flTF beschrieben, die durch ein alternatives Splicing entsteht. Seit seiner Entdeckung
wird über die Rolle des alternativ gespleißten
Tissue-Faktors (asTF) bei der Gerinnung diskutiert. Der asTF könnte gerinnungsfördernde
Eigenschaften besitzen, aufgrund struktureller Unterschiede im Vergleich zu flTF ist die
Gerinnungsfunktion bei asTF jedoch möglicherweise relativ gering. Allerdings scheint
asTF, ähnlich wie flTF, nicht hämostatischen
Eigenschaften zu besitzen; die Expression von
asTF in Tumoren korreliert positiv mit der Tumorgröße, der Anzahl der Gefäße und einem
schlechteren Überleben bei bestimmten Tumorarten und verstärkt in Tiermodellen das
Tumorwachstum. Interessanterweise fördert
asTF im Gegensatz zu flTF die Angiogenese
nicht über die Aktivierung der PARs, sondern
vielmehr über die Ligation von Integrinen. Von
zentraler Bedeutung ist flTF bei kardiovaskulären Erkrankungen, man weiß jedoch wenig
über die Rolle von asTF bei diesen Erkrankungen. asTF wird in Monozyten und Makrophagen gebildet, daher könnte asTF mit makrophagealem Ursprung zu arteriosklerotischen
Erkrankungen beitragen. Abschließend kann
man sagen, dass die Erforschung der nicht-hämostatischen Eigenschaften des asTF neue
Einblicke in die Pathophysiologie und Diagnostik von Krebs und kardiovaskulären Erkrankungen schaffen könnte.
and when activated, Xa leaves the
flTF:FVIIa complex and then converts prothrombin into thrombin. Finally, fibrin
fibers are generated, which together with
activated platelets, form a haemostatic
plug. The pattern of flTF-expression thus
serves as a haemostatic envelop surrounding the vasculature (1).
It was long thought that under physiological conditions flTF is not in contact
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Y. W. van den Berg; H. H. Versteeg: asTF
with the blood. However, in the late nineties circulating TF antigen in blood was
first detected (2). Further studies revealed
that levels of circulating flTF antigen, either
on the surface of microparticles or on activated monocytes, are higher in plasma from
patients suffering from diseases such as coronary artery disease, sepsis, cancer or sickle
cell disease (3–6). In addition to these circulating forms of TF, Bogdanov described an
alternatively spliced soluble isoform of flTF
(asTF) in 2003 (7). This TF isoform results
from alternative splicing of the flTF transcript, in which exon 5 is omitted. As a result,
a shift of the open reading frame occurs and
the transmembrane and cytoplasmic domains are replaced with a unique 40 amino
acid C-terminal domain, rendering asTF
soluble. Later, the same group identified murine asTF in which exon 5 is also omitted as
a result from alternative splicing (8). Since its
discovery the role of asTF in coagulation has
been a matter of debate. Some reports attribute procoagulant function to asTF and
find asTF present in thrombi, but others exclude such a role for asTF, making the role of
asTF in physiology unclear. More recently,
asTF was shown to be expressed in tumour
tissues and to have angiogenic properties,
suggesting a role in tumour angiogenesis.
Although the majority of the research on
asTF in (patho)physiology has focused on
cancer, it is becoming clear that asTF might
also impact on cardiovascular disease.
In this review we will discuss the current
literature on asTF considering its synthesis
and its role in (patho)physiology.
TF pre-mRNA splicing
In biology the process of pre-mRNA splicing
is a carefully regulated process and alternative
splicing increases the proteome’s abilities to
adapt to environmental changes. Serine/arginine rich proteins (SR proteins) regulate constitutive and alternative splicing of premRNA depending on their phosphorylation
status by binding to exonic splicing enhancer
(ESE) motifs. In silico analysis revealed that
exon 5 of the TF gene contains ESE motifs that
bind SF2/ASF and SRp55, which are SR proteins that are abundantly expressed in monocytic cell lines and monocyte-enriched peripheral blood mononuclear cells.
Use of a minigene reporter system that
was transiently expressed in monocytic
cells, led to the identification of five ESE
motifs in exon 5 that bind SF2/ASF and one
ESE motif binding to SRp55. Selective mutagenesis of these ESE motifs weakened the
SR protein-RNA binding, which further
supported that SF2/ASF and SRp55 regulate TF pre-mRNA splicing in monocytes
(9). Further analysis of the monocytic
THP-1 cell line demonstrated the presence
of two other SR proteins, SC35 and SRp40,
both having binding sites in exon 5. However, in stead of exon inclusion, SC35 and
SRp40 were found to promote exon exclusion by competing with SF2/ASF and
SRp55 (10). A delicate balancing of SR protein function underlies TF pre-mRNA
splicing; however, the mechanism how SR
protein phosphorylation status is regulated
in monocytes is still unknown.
As in monocytic cells, TF pre-mRNA
processing has also been studied in endothelial cells and furthermore, upstream
signaling events in SR protein phosphorylation have been characterized. Human
umbilical vein endothelial cells (HUVECs)
are shown to express and secrete asTF after
stimulation with TNF-α leading to pro-coagulant activity of the culture medium after
addition of phopholipids (11). The stimulus with TNF-α was found to affect the
phosphorylation status of SR proteins that
are present in HUVECs, SRp75, SRp55 and
SF2/ASF. After pharmacological interference with Cdc2-like kinase, the phosphorylation status of the SR proteins, SRp75,
SRp55, and SF2/ASF was diminished in
TNFα-stimulated cells. In a similar approach, inhibition of DNA topoisomerase I
down-regulated phosphorylation of SRp55
and SF2/ASF, while leaving SRp75 unafTab. 1
Overview of SR proteins determining
exon 5 inclusion or exclusion in TF pre-mRNA
splicing in monocytes and endothelial cells
exon 5
inclusion
exclusion
monocytes
SF2/ASF
SRp55
SC 35
SRp40
endothelial
cells
SF2/ASF
SRp55
SRp75
unknown
fected. Experiments in which SRp75 and
SF2/ASF were directly targeted with siRNA
revealed that these SR proteins predominantly regulate the balancing between flTF
and asTF mRNA. Whereas silencing of
SF2/ASF up-regulated asTF mRNA levels,
those of flTF mRNA were diminished; silencing of SRp75 left asTF mRNA levels unaffected, but lowered flTF mRNA levels.
Changes at the mRNA level of these experiments indeed resulted in a diminished procoagulant activity of the HUVECs indicating the importance of TF pre-mRNA splicing in coagulant activity of stimulated HUVECs (12). In addition to Cdc2-like kinases
and DNA topoisomerase I, the PI3K/PKB
(Akt) pathway, but not the NF?B pathway,
was reported to alter phosphorylation
status of SRp55, SRp75 and SF2/ASF in endothelial cells upon TNF-α stimulation
(13). The effect of SR proteins on exon 5 inclusion or exclusion in endothelial cells and
monocytes are summarized in 씰Table 1.
The studies on TF pre-mRNA splicing in
endothelial cells and monocytes provide a
better understanding of vascular wall thrombogenicity and lead to the plausible theory
that pathological conditions could promote
asTF synthesis in endothelial cells and monocytes. Whether this could lead to a prothrombotic state and how TF pre-mRNA
splicing might be altered in e. g. cancer are
questions that remain unanswered.
Coagulation and asTF
As mentioned, asTF differs significantly from
flTF such that a major part of the TF extracellular domain is replaced by a new C-terminus, which largely impacts on coagulant
function. Nevertheless, Bogdanov and coworkers reported asTF to have pro-coagulant
activity in presence of phospholipids. In addition, they showed asTF to be incorporated
at the growing edge of in vivo thrombi. This
led to the theory that asTF has its main
physiological role in propagating thrombus
formation (7). Szotowski and colleagues proposed that endothelial cells may act as a
source for asTF after stimulation with IL-6
and TNF-α. Furthermore, asTF produced by
HUVECs supported generation of activated
factor X (FXa) in presence of phospholipids,
which supports the theory that pathologic
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Y. W. van den Berg; H. H. Versteeg: asTF
conditions trigger asTF production in endothelial cells hereby leading to a pro-thrombotic state (11).
However, several studies by others failed
to show pro-coagulant activity of asTF
(14–16). Censarek and Hobbs used cells
that were transfected with an asTF-expression construct, an in vitro set-up that
differs from cytokine-stimulated HUVECs
and that therefore may account for their
findings. In addition, Böing found asTF to
be retained in cells thereby limiting the
bioavailability of asTF; however, in her experiments HUVECs were stimulated with
IL-1a which may be a rather mild stimulus
compared to TNF-α used in previous experiments. Nevertheless, these data seriously question the pro-coagulant activity
of asTF. Despite the presence of the functional the 165–166 lysine doublet in asTF,
which is critical for binding to VIIa, the affinity of asTF for VIIa is most likely diminished when compared to that of soluble
flTF, consisting of the complete extracellular domain (AA 1–218). Furthermore, due
to absence of the AA residues encoded by
exon 5, asTF might lack a proper X docking
domain explaining its hampered properties
in promoting blood coagulation. Moreover, it is unclear how a proposed
asTF:FVIIa complex can associate with the
phospholipid surface which is essential for
an efficient conversion of X into Xa (17). As
reviewed before, the contradictory findings
on coagulant activity of asTF may be due to
differences in experimental set-up and
therefore more work is needed to elucidate
the role of asTF in coagulation (18).
TF:FVIIa signaling produces expression of
VEGF, IL-8 and metalloproteinases in vitro
and CXCL-1 in vivo (23). tumour cell signaling by the flTF-VIIa-PAR-2 axis is
negatively regulated by the flTF cytoplasmic tail, but phosphorylation of this domain, which results from activation of
PAR-2, reverses this inhibition (24). In addition, flTF physically associates with α3β1
and α6β1 integrins and disruption of this
complex using a flTF monoclonal antibody,
downregulates pro-angiogenic signaling
and resulting tumour growth in vivo (23,
25). Importantly, flTF can exist in an reduced and oxidized form, and whereas the
reduced signaling form appears to be involved in PAR-2-dependent tumour angiogenesis, the oxidized form influences metastasis via activation of the coagulation cascade and activation of PAR-1 (23, 26). Thus,
flTF influences various stages in the cancerous process, depending on its interaction
with other proteins and its redox state.
Cancer and asTF
In multiple types of cancer high tumoural
flTF-expression and thrombotic complications are described and the role of the
TF:FVIIa complex in cancerous processes
has since long been recognized (19–21).
Knockdown of flTF in colorectal cancer
cells xenografted in mice resulted in impaired tumour angiogenesis and consequently tumour growth (22). It is currently thought that flTF together with its
ligand factor VIIa, regulates the angiogenic
switch in tumours through activation of
Protease-activated Receptor-2 (PAR-2).
Fig. 1 asTF promotes angiogenesis ex vivo:
asTF angiogenic properties were tested in an aortic ring assay . Aortic segments from C57Bl6 mice
were embedded in matrigel supplemented with
solvent control (upper panel) or asTF (lower
panel). Pictures are shown of outgrowing sprouts
on day 5.
Despite this prominent role of TF:FVIIa
signaling in cancer, it is becoming increasingly clear that asTF might be an equally important modulator in cancerous processes.
Several groups found asTF to be expressed in
both tumour cell lines and tissue specimens
from cancer patients. 8 out of 9 pancreatic
cancer cell lines were reported to express
asTF and pro-coagulant properties of the
culture medium were attributed to presence
of asTF as well as shedding of flTF-bearing
microparticles (27, 28). However, experiments in colorectal cancer cell lines indicated that microparticle-bound flTF rather
than asTF may contribute to the pro-thrombotic state in cancer patients (29).
Although a causal role of asTF in cancerassociated thrombosis has not been established yet, asTF expression was shown to relate to clinical outcome in non-small cell
lung carcinoma (NSCLC). Lower asTF
mRNA levels were found in NSCLC patients with grade Ia disease when compared
to patients with a more advanced staging
and in concordance with these findings, tumoural asTF expression related to a poorer
prognosis in NSCLC in another study (30,
31). In contrast to NSCLC, in oesophageal
cancer, asTF mRNA levels were comparable
with healthy controls, whereas flTF expression was increased in tumour samples
(32). Further, our group detected intratumoural levels of asTF ranging from 0–75
nmol/l in cervical cancer specimens and
found asTF to be differentially expressed
but whether this relates to clinical outcome
is still unclear (33). Apparently, flTF and
asTF expression patterns may differ
amongst tumour types and how asTF expression relates to clinical outcome and
whether asTF may be used as a bloodborne marker for staging cancer remains to
be investigated.
Despite presence of asTF mRNA and
protein in tumours, the mechanism how
asTF affects cancer is still elusive. Hobbs
and colleagues set out to investigate how
tumoural asTF expression influences tumour biology by means of an in vivo
model. They used the pancreatic cancer cell
line MiaPaca2, which does not express endogenous TF to generate cells that selectively express flTF or asTF expression by
means of stable transfection. The flTF-expression construct conferred pro-coagu-
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lant TF activity to the culture medium but,
in line with earlier observations, the asTFexpression construct failed to do so. When
cells were injected subcutaneously to assess
tumour growth, flTF expression rendered
smaller tumours when compared to cells
that did not express TF, but asTF expression
increased tumour size. Immunohistochemical analyses of the tumours revealed
that asTF-expressing tumours display
more tumour vessels and vessel leakage, a
feature of tumour angiogenesis. However,
these experiments failed to assess how asTF
expression results in increased angiogenesis. One option is that asTF drives tumour
growth, resulting in tumour cell-dependent angiogenesis through secretion of proangiogenic factors. Alternatively, asTF may
directly impact on the formation of blood
vessels which subsequently drives tumour
expansion. To uncover the exact role of
asTF in tumour angiogenesis our group set
out to characterize the effect of recombinant purified asTF on in vivo and ex vivo
angiogenesis. Using a matrigel plug assay in
which mice were injected subcutaneously
with matrigel containing either 100 nM
recombinant human asTF, 50 ng/ml recombinant murine VEGF or buffer control,
we could show that asTF induces an angiogenic response comparable to that evoked
by VEGF. In line with these findings we
found that human recombinant asTF was
able to significantly promote angiogenesis
at levels as low as 1 nmol/l in a murine aortic ring assay (씰Fig. 1). Since flTF exerts its
angiogenic effect through binding to VIIa
and subsequent PAR2 signaling, we first investigated whether asTF could facilitate
PAR2 signaling in endothelial cells. To this
end, endothelial cells that were adenovirally transduced in order to express PAR2
were stimulated with asTF alone, asTF in
combination with FVIIa or a PAR2 agonist
as positive controls. Whereas the PAR2
agonist elicited a potent phosphorylation
of p44/p42 MAPK, asTF alone and the
combination of asTF and VIIa failed to do
so, which led us to conclude that asTF-induced angiogenesis is not PAR2-driven.
Then we dissected the mechanism of asTFinduced angiogenesis into stages of endothelial cell adhesion, migration and capillary formation. All three processes could
be triggered with asTF and were sensitive to
Fig. 2 asTF is present in inflammatory infiltrates in atherosclerotic plaques: Paraffin embedded specimens from atherosclerotic plaques
on microscope slides were deparaffinized and
rehydrated. Endogenous peroxidase activity was
blocked with 0.3% H2O2 in methanol. For flTF
stainings, slides were subjected to antigen retrieval by cooking for 10 minutes in sodium citrate
buffer. Antigen retrieval was omitted for asTF
stainings. After blocking with 10% normal goat
serum in PBS/BSA 1%, slides were incubated with
an asTF-specific antibody at a concentration of 1
μg/ml or flTF antibody at a concentration of 10
μg/ml. After appropriate washing steps and incubation with a secondary antibody, immunostaining was performed with 3,3'-Diaminobenzidine
(DAB) peroxidase. Sections were counterstained
with haematoxylin.
blockade with specific antibodies to integrin subunits. Whereas asTF-induced endothelial cell migration depended on integrin αVβ3, capillary formation could be
abrogated by integrin α6β1 blockade. Interestingly, asTF was shown to ligate different integrins than flTF, indicating that asTF
may have a different role in non-haemostatic processes than flTF.
After binding to asTF coatings, endothelial cells displayed phosphorylation of
Focal Adhesion Kinase (FAK), PI3 kinase,
p38 MAP kinase, p44/42 MAP kinase and
Akt. By use of specific kinase inhibitors we
found that asTF-induced cell migration de-
pended on PI3 kinase and p38 MAP kinase,
but not on p44/42 MAP kinase. Use of an
Akt inhibitor only modestly inhibited
asTF-induced endothelial cell migration
which indicates that both Akt-dependent
and independent pathways are involved in
asTF-induced endothelial cell migration.
In a similar approach we identified PI3 kinase and p44/42 MAP kinase, but not the
promigratory p38 MAP kinase to be crucial
in asTF-induced capillary formation. In
support of that, cells allowed to form capillaries on matrigel supplemented with asTF
showed higher levels of p42/p44 and Akt,
but not p38 phosphorylation (33).
So far it is known that asTF is expressed
in several tumour types and that its expression may relate to clinical outcome. In
addition, the intratumoural asTF concentration is sufficient to promote angiogenesis through integrin ligation independent
from PAR2-signaling. However, more research is needed to determine the relationship between tumoural asTF expression
and circulating asTF levels in cancer patients on the one hand and clinical and tumour type characteristics on the other. In
addition, further studies are needed to
elucidate the mechanisms that trigger asTF
production in tumours.
Cardiovascular disease
Apart from inflammation and cancer, flTF
also influences cardiovascular disease. Levels of flTF expression are lower in patients
with dilated cardiomyopathy (DCM) hereby influencing cell-to-cell contacts and
myocardial contractility. In parallel with
flTF expression, asTF mRNA levels were lowered in these patients as well, but the pathophysiological relevance of asTF lowering in DCM patients is still unclear (34).
Diabetes greatly influences the biology of
cardiovascular disease and flTF in pancreatic
islet cells has been proposed to be an intermediate between coagulation and cardiovascular disease. During prolonged hyperglycemia alpha and beta cells in the islets of
Langerhans express higher levels of flTF and
they secrete microparticle-bound flTF and
asTF. Whether islet cell-derived circulating
TF indeed contributes to atherothrombosis
is unclear. Furthermore, whether asTF can
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Y. W. van den Berg; H. H. Versteeg: asTF
influence the endocrine pancreatic microenvironment is even more speculative (35).
Lipid rich necrotic cores of atherosclerotic plaques contain flTF which takes
part in atherothrombus formation after
plaque rupture. Furthermore, high levels of
the circulating TF antigen are present during
both acute and chronic phases of atherosclerotic disease (36, 37). However, to what
extent asTF contributes to circulating TF
antigen levels in cardiovascular disease is unknown. The same applies to the role of asTF
in atherosclerotic plaques. Activated monocytes have been found to be bona-fide producers of asTF (10), making a role for asTF
in cardiovascular disease plausible. A critical
step in the formation of an atherosclerotic
plaque is the invasion of monocytes into the
subendothelium, followed by their transformation into foam cells which eventually
undergo necrosis. As a consequence, monocyte-derived asTF may become available in
atherosclerotic plaques to either stimulate
vasa vasorum formation, an event that has
been shown to disrupt the integrity of the
atherosclerotic plaque (38).
In preliminary experiments we stained
atherosclerotic plaque material for asTF
and we found inflammatory infiltrates to
stain positive for asTF (씰Fig. 2), confirming that asTF might play such a role. However, to establish a relationship between
asTF and plaque angiogenesis, extensive
analysis of human atherosclerotic plaque
material for asTF expression and angiogenic markers will prove vital.
Future research
Despite the work that has been done on asTF
function, the role of asTF in (patho)physiology is still unclear. Primary cell cultures and
cell lines have been helpful for identification
of asTF mRNA in cancer and the study of the
splicing mechanism of TF pre-mRNA.
However, the mechanism of alternative
splicing in cancer is still unclear and cancer
cell lines offer the possibility to study this
mechanism. However, cell culture has some
serious limitations considering the study of
(patho)physiologic mechanisms. Lack of
proper triggers or the interplay between different cell types might explain the problems
that are encountered by researchers when
trying to have asTF secreted in culture media
from cells in culture.
Therefore in vivo models may be helpful
to obtain more insight in the functions of
asTF. Our group showed that murine cell
lines as well as mice respond to human recombinant asTF in experiments on angiogenesis which justifies the study of human
asTF in a murine system. However, the field
would benefit from a functional characterization of murine asTF in order to enable the
use of valuable in vivo models that could be
performed in a complete syngeneic approach. Moreover, the generation of murine
models on asTF might prove valuable. Although mice have been generated that express low levels of TF and essentially are
knock-outs for asTF, it is hard to interpret
how the loss of asTF affects the phenotype of
these mice (39).
Another direction of future research
that may improve our understanding of the
relevance of asTF in pathology is determining the relationship between asTF expression and clinical data in cancer. At present the relation between asTF expression
and clinical staging is only investigated in
non-small cell lung cancer and esophageal
cancer on the mRNA level. It is possible that
asTF has different effects in different types
of cancer, which necessitates the study of
asTF in other cancers, also focusing on protein level. Associations found in such
studies will give important clues to direct
future mechanistic studies. Furthermore,
the detection of circulating asTF protein in
plasma samples from these patients will be
helpful and may even serve as a biomarker
for certain types of cancer. As in cancer,
asTF may have biomarker properties for
patients suffering from cardiovascular disease, however, the role of asTF in cardiovascular disease should be clarified first.
Conclusion
Since its discovery the (patho)physiologic
role of asTF has been unclear. Compared to
flTF, asTF seems to have only weak pro-coagulant properties. Use of different in vitro
approaches hampers a clear interpretation
of the experiments done so far on the role of
asTF in coagulation. In addition, future
studies may elucidate which triggers are
necessary for asTF secretion from cells. Although debated as a cripple protein in blood
coagulation, asTF seems to have interesting
non-haemostatic properties in cancer. Future research is needed on asTF and is likely
to provide more insight in the role of TF isoforms in tumour biology and cardiovascular
disease. In addition, its properties as a soluble circulating protein may be beneficial for
the development of new diagnostic tools.
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Y. W. van den Berg; H. H. Versteeg: asTF
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